Tom Baker
Meglos

The Doctor is invited by the leader of Tigella, Zastor, to visit the planet because there is concern between the religious Deons and the scientific Savants. The TARDIS is intercepted by Meglos, the megalomaniacal last survivor from the planet Zolfa-Thura.

The TARDIS is trapped in a time loop, although the Doctor manages to break free. Meglos uses the stolen body of an Earthling and transforms himself into a duplicate of the Doctor.

Disguised as the Doctor Meglos steals the Tigellans' power source - a mysterious dodecahedron. Meglos plans to use the dodecahedron to power an apocalyptic device which would be used to destroy Tigella.

The Doctor is hindered by the Gaztak, hired by mercenaries, but manages to succeed in tampering with the dodecahedron, and the resulting explosion destroys Meglos.